My prejudice is that not that many roses bred on the West Coast that would survive in England. Not without some extra work. One of the observations I made while growing roses in NJ was that most roses bred in Germany and England were more resistant to black spot than roses bred in California. So I formed the impression that English and German rose breeders put more energy into breeding cultivars resistant to fungal disease than Californian rose breeders. That's possibly something that has changed a bit in the last decade or two. I think many of Carruth's introductions, for example, address disease issues.
One of the great motivations of David Austin's breeding program was to develop a line of roses that survived happily in the British weather without much spraying - a fact that suggests to me that disease among hybrid teas and floribundas bred for the British Isles before his work was considered to be a significant problem when he started in the sixties and seventies.
The coolness of English summers, I think, can explain why they might not have to worry nearly so much about blackspot as, say, rose growers in Arkansas. But that same coolness promotes powdery mildew just as it does along the coast of CA.
It seems to me that roses with a stronger dose of multiflora heritage might be a little more prone to mildew, on average, than those that have less. And roses with dull leaf surfaces seem to suffer from mildew a little more, on average, than those with shiny leaf surfaces. FWIW. In any case, I would expect most David Austin roses to fare pretty well in coastal CA. Even at the height of my mildew outbreak, I have not observed any mildew on any of my DA roses here; but I do not have Pat Austin.
Finally, I notice that in the case of powdery mildew, it seems to be the new fresh leaves that get attacked first. The old established leaves seem to be a little more resistant. I guess this is one of the reasons people suggest that overfeeding roses can promote mildew.
